3 Answers
3

If this is only happening when the vehicle is in motion, I'd visually trace the wiring and look for a point where a wire or wire bundle is rubbing on something and has worn through the insulation.

If it doesn't exhibit the problem when it's parked, it's going to be hard to track down with a multimeter or test light. Unless you can reproduce it by gently manipulating parts of your wiring harness.

You'll want a wiring diagram and a troubleshooting tree, which will probably be in the factory service manual. Diagrams showing connector and wire bundle locations would be nice, too (and quite possibly also in the FSM). Other manuals will have less detailed information, but maybe enough to get you by.

You'll want to check the parts of the wiring you can get at without too much trouble and try and localize the fault. If it's in an an inaccessible location, it may be easier to splice in a new length of wire and route it as best you can around the bad spot. Especially if your wire of interest is in the middle of a wire bundle.

If it's a fused circuit, you should at least be able to narrow it down to which side of the fusebox the problem is on, assuming you can reproduce the short at will (don't try anything while the vehicle is in motion, you'll just win a darwin award).

If you're looking for a short, set your multimeter to continuity (ohms) and stick the probes on the wire you want to check (when the circuit is not powered). A small nonzero number means you have a connection between the probes. Check this tutorial.

This is almost certainly one of the door 'pin switches' out of adjustment.
Try making up a pad of some sort where the door shuts onto the switches.
I have used stick on wheel weights quite effectively for this!.

For a preliminary inspection, I would arm the alarm, then walk around and pull on each door. If it doesn't go off, then you're probably looking at wiring. Bang on the metal of each door and see if you can make the light come on. Tug in the harness under the dash and the lower side kick panels in front of the doors and under the dash.

The two most common areas for this problem are wires inside the door cutting on the window regulator, and under the front door sill plates in front where wires to the back doors typically run